Aachen PK 2003 – scientific programme

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PV: Plenarvorträge

PV V

PV V: Plenary Talk

Tuesday, March 18, 2003, 14:00–14:45, FO1

Controlling rotation and exhaust in the tokamak edge — •Per Helander — UKAEA, Culham Science Centre, Abingdon OX14 1DQ, UK

The edge region has a pivotal effect on tokamak plasma performance. In this talk, two techniques to control its behaviour are described. Both schemes were originally suggested by theoretical considerations, and have subsequently been tested on MAST. The first one concerns the influence of neutral atoms on edge plasma rotation. Neoclassical theory of ion-neutral interaction through charge exchange suggests that the radial electric field and plasma flow in the tokamak edge should depend sensitively on the poloidal location of the atoms. In a collisional plasma, the toroidal rotation, electric field, and their shear should be largest if the atoms are localised on the inboard side of the torus. This appears to be confirmed by rotation measurements on MAST, and may help to explain why H-mode access is much easier with inboard gas puffing. The second topic concerns experiments where toroidally alternate divertor plates are electrically biased in an attempt to to broaden the SOL and reduce the divertor heat load. As suggested by theory, this has a large effect on the divertor plasma but does not interfere with core plasma performance. A range of phenomena are observed during biasing, including dramatic strike-point movement and broadening, SOL heating, and changes in edge rotation.